what is redox couple
Defined as having together the oxidised and reduced forms of a substance taking part in oxidation or reduction half-reaction.
Represented by separating the oxidised and reduced forms by a vertical line
For example, when zinc rod is dipped in CuSO4 solution, zinc is oxidised to Zn2+ ions and Cu2+ ions get reduced to metallic copper.
Here, redox couples are Zn2+/Zn and Cu2+/Cu.
The representation of cell is
Zn/Zn2+||Cu2+/Cu. Look the following fig. for this.